Wrought nickel superalloys of the gamma-gamma prime type are widely used in modern gas turbines. One such particular component is the disk in the turbine section which rotates at high speeds and elevated temperature. The disk is subjected to very high stresses and has a great amount of stored energy. It must therefore be designed and fabricated in a manner which avoids failure to the greatest extent possible. Naturally, only the finest materials and most careful processing are used. Repetitive inspections to the highest technical level possible are used to ensure quality.
Of particular concern is the possibility of minute surface cracks which can propagate over time and result in metal fatigue failure. Therefore, various surface inspection procedures are used to find such minute cracks, including ultrasonic, eddy current and fluorescent penetrant inspection (FPI) techniques. Defects which are located within about 1.3 mm of the surface of such articles are especially difficult to find. Eddy current and fluorescent penetrant inspection are preferred. And since it is well known that all these processes have their advantages, disadvantages and reliability limits, it is not uncommon that a critical article such as a gas turbine disk be subjected to redundant inspections.
Those familiar with inspection technology are well aware that the degree of uncertainty rises when the limits of detectability are approached. Yet, it is highly undesirable that costly components be discarded because of a spurious inspection signal. Consequently, confirmation is generally sought between the independent methods.
In the particular instance of inspecting articles made of the nickel alloy IN-100 (an alloy of Inco, Ltd, New York), problems have been encountered in obtaining such verification. For example, eddy current inspection may indicate the presence of a flaw, whereas there will not be the expected confirmation from FPI. Experimental work has shown that this can be attributable to the smearing over of minute cracks on the metal surface due to conventional machining. When the exposed surface of a crack has metal smeared across it, then the fluorescent penetrant cannot enter the crack, and there is no confirmation.
Therefore, the problem has been how to best improve the present methods of inspection.